hwmon: enhance the sysfs API for power meters

Augment the documentation of the hwmon sysfs API to accomodate ACPI power
meters and the current desired behavior of power capping hardware drivers.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Len Brown <len.brown@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>